The present invention relates to crystalline and amorphous films produced by line-of-sight low pressure (non-scattering) processes such as evaporation or sputtering in which molecular or atomic-size particles are removed from a target of a selected source (target and source are used interchangeably herein) by either the use of thermal energy in the case of evaporation or the use of controlled ion bombardment in the case of sputtering and deposited on a substrate to form a coating or film of the source material on the substrate.
The deposited films may exhibit columnar microstructures, with the details of the microstructure depending on deposition parameters such as with respect to sputtering the nature of the adatom flux (energy distribution, density, composition, angle of incidence etc.) and the conditions at the substrate surface (nucleation sites, surface finish, impurities, surface temperature etc.). It is probable that geometric shadowing is involved in the development of these columnar growth features, with the columnar structure being more pronounced with increasing proportion of low angle incidence adatoms and with increasing surface roughness providing more effective shadowing sites. The bondaries between these columnar regions or growth sites may be gaps between isolated growth structures resulting in a reduced deposit density. It is desirable for preparing many materials, for example, certain magnetic materials or materials useful for their magnetic properties, certain semiconductor materials, and materials useful for their optical properties, to provide coatings which are close to theoretical density.
Sputtering is the process of removing molecular or atomic-size particles from the target by controlled ion bombardment. In the case of triode sputtering, the target or source and the substrate are separately formed from the anode and cathode. The system is placed in a low-pressure, inert atmosphere and with the anode held positive with respect to the cathode, electrons emitted by the cathode are attracted toward the anode. The release of electrons is accomplished by placing a high potential between the anode and the cathode or by using a heated or therminoic cathode. The electron current between the cathode and the anode causes a gas discharge, or plasma, to form in the space between electrodes, due to ionization of the inert gas. The plasma consists of a cloud of negative electrons and a similar cloud of positive ions leaving the plasma substantially neutral.
The target and the substrate are placed in the plasma, and the target is maintained at a potential negative with respect to the ionized plasma. This serves to attract the positively charged plasma ions toward the target. These ions arrive perpendicular to the target surface at a level of energy equivalent to the target potential. This bombardment dislodges molecular or atomic-size particles of the target material by momentum transfer processes. There sputtered atoms are moving rapidly (kinetic energy on the order of a few electron volts) and many of them strike the substrate where they form a deposited coating or film. Ejection from the target surface is such that in general (certain alloys and packed crystal structure excepted), flux density is proportional to the cosine of the angle of ejection and is maximum normal to the target surface and minimum parallel to the target surface.
Previously, dense coatings have been obtained by positioning the selected source or target material substantially parallel to and generally opposite the selected substrate material while maintaining the area of the selected target or source material relatively small in order to limit the shadowing due to particles impinging upon the substrate at varying angles of incidence. One of the problems with this method is low deposition rates resulting from low flux densities. Generally, the target surface area cannot be increased to increase the flux density without incurring increased columnar defects. Increasing the energy to the target in order to increase flux density can result in melting the target surface or overheating the substrate or both.
As the target area is increased in an attempt to obtain a higher flux density there is an increase in the shadowing due to the arrival of particles at the substrate surface at varying angles of incidence leading to more or increased columnar defect shadowing, that is defect structure or leaders parallel to the angle of incidence of the incoming particles. Retaining the parallel target and substrate configuration along with maintaining a relatively small target area inevitably results in low flux densities. Flux density also cannot be increased by increasing the power input because for sputtering, which is a momentum transfer phenomenon, only a small amount of the energy of the incident ions is used to dislodge molecular or atomic-size particles from the target, the rest of the energy being converted to heat. Therefore, with sputtering, target cooling capabilities in large part determine the maximum acceptable power input.
It is possible to use high substrate temperatures to increase the mobility of the atoms being deposited in order to obtain high density deposits of thin films rather than attempting to limit the angles of incidence of the deposited particles. However, it is often desirable to retain deposits in a form produced by low substrate temperatures, resulting in fine grain size and uniform composition. High substrate temperatures would preclude this. Further, high substrate temperatures may result in other undesirable effects such as alloying reactions between the substrate and the material being deposited.
Accordingly, for many purposes in order to obtain close to theoretical density coatings with line-of-sight (low pressure non-scattering) processes the customary target to substrate configurations necessarily result in low deposition rates due to poor flux densities.
U.S. Pat. No. 3,494,852 issued Feb. 10, 1970 to M. Doctoroff for A Collimated Duoplasmatron-Powered Deposition Apparatus relates to production of duoplasmatron ion beam used to deposit high density film on a substrate.
U.S. Pat. No. 3,494,853 issued Feb. 10, 1970 to D. E. Anderson et al. for Vacuum Deposition Apparatus Including a Programmed Mask Means Having a Closed Feedback Control System relates to a universal mask system for depositing a material from the souce or target at a preselected area of the substrate.
U.S. Pat. No. 3,654,123 issued Apr. 4, 1972 to Hajzak for Sputtering relates to a device to achieve controlled coatings by sputtering in which continuous relative movement between the plasma and the substrate is achieved.
U.S. Pat. No. 3,669,860 issued June 13, 1972 to Knowles et al. for Method and Apparatus for Applying a Film to a Substrate Surface by Diode Sputtering discloses a diode sputtering apparatus including means for cooling the substrate to prevent overheating during deposition.
U.S. Pat. No. 3,761,375 issued Sept. 25, 1973 to J. T. Pierce et al. for Process For Fabricating Vidicon Tube Target Having A High Resistance Sputtered Semi-Insulating Film discloses an R.F. sputtering process for semiconductor substrates in which the high resistance deposited film functions as a charge carrier path.
U.S. Pat. No. 3,779,891 issued Dec. 18, 1973 to Vegh et al. for Triode Sputtering Apparatus discloses a target material including two members with the surfaces thereof angularly disposed.
U.S. Pat. No. 3,829,373 issued Aug. 13, 1974 to M. R. Kuehnle for Thin Film Deposition Apparatus Using Segmented Target Means shows a device with the substrate means arranged on the exterior of a cylindrical drum for rotation with respect to circumferentially spaced arcuate target segments coaxially arranged with the drum.
U.S. Pat. No. 4,014,779 issued Mar. 29, 1977 to M. R. Kuehnle for Sputtering Apparatus discloses a sputtering machine having a rotary anode insulated from the supporting structure and maintained at a voltage differential with respect to the supporting structure.
U.S. Pat. No. 4,119,881 issued Oct. 10, 1978 to Caldron for Ion Beam Generator Having Concentrically Arranged Frustoconical Accelerating Grids discloses a grid system for an ion beam generator having a frustoconical shape so that the collimated ion beam converges at a predetermined angle to provide selective beveled etching of the target.